This invention relates to the field of millimeter wave imaging systems and, more particularly, to millimeter wave imaging systems using two dimensional focal plane sensor arrays.
Millimeter waves are another term for radio waves sent at higher frequencies. Millimeter waves are in the region of the light spectrum between 30 GHz and 300 GHz.
In the field of imaging, active radar at longer wavelengths and infrared and optical systems at shorter wavelengths are more mature technologies. Passive millimeter wave sensing is a newer technology taking advantage of the fact that millimeter waves can penetrate inclement weather and opaque solids. Work on passive millimeter wave sensing expanded with the use of microwave and millimeter wave integrated circuit (MMIC) technology that allows sensing in this region using small integrated chips. Scanners and imagers can be used for sensing at 35, 94, 140, and 220 GHz.
Certain parameters are used to determine sensitivity levels of an imaging sensor. For example, Noise Equivalent Power (NEP) is the minimum detectable power per square root of the bandwidth and Noise Equivalent Delta Temperature (NEDT) refers to the change in temperature that yields a signal-to-noise ratio of unity in a thermal imaging system.
Millimeter wave imagers have been under development for the last couple of decades. Millimeter wave imagers are useful for a number of functions, including concealed weapons detection, for example, for airport security, battlefield imaging through smoke and dust, imaging through visually opaque objects like walls, biohazard detection, and vehicular driving and landing aids. Given the potential for high volume civilian use, for example in airport security and as vehicle driving aid, there is a need for effective millimeter wave imagers.
In the field of millimeter wave imaging systems, two-dimensional sensor arrays can be used instead of a film when placed in the focal plane of a camera lens. The two-dimensional sensor arrays are capable of detecting the incoming radiation in shades of grey based on the intensity of the incident rays. These focal plane arrays have been used to detect thermal radiation which is normally quite low. An antenna detects the radiation and the detected radiation is amplified. In an exemplary 100×100 array of sensors, the same large number of amplifier chips are used, the incident radiation being detected after amplification.
However, a number of characteristics have resulted in high sensor cost. For example, many RF detectors have required cooling. See Development of a 3×3 Micromachined Millimeter Wave SIS Imaging Array, Lange, et, al., IEEE Trans. on Applied Superconductivity, vol 7, No, 2. June 1997. Other detectors do not provide the required responsivity. See Active Millimeter-wave Video Rate Imaging With a Staring 120-element Microbolometer Array, Arttu Luukanen, Aaron J. Miller, and Erich N. Grossman, Proc. SPIE Int. Soc. Opt. Eng. 5410, 195 (2004). Yet, other sensors, for example those including Schottky diodes, do not provide low noise and their noise is proportional to their required bias current. See Terahertz Technology, P. Siegel, IEEE Trans. Microwave Theory and Techniques, Vol. 50, No. 3, March 2002. Finally, most sensors have been difficult to manufacture, for example Ge diodes, or have exhibited strong temperature dependence.
These limitations have resulted in the use of radio frequency (RF) components, such as RF low noise amplifiers (LNAs) and RF Dicke switches, in order to achieve acceptable performance levels. RF LNA and RF Dicke switch components, however, are expensive and full two dimensional focal plane arrays that could contain thousands of pixels have been extraordinarily expensive. LNA elements also generate excessive heat such that using a large array of these elements close to one another becomes impracticable.
To help mitigate the high costs of two dimensional focal plane sensor arrays, some millimeter wave imagers are implemented in a scanning mode so that only a single element, or at most a single row of elements, is needed and the scene is produced through scanning. U.S. Pat. No. 6,417,502 to Stoner presents one example of millimeter wave scanning imaging systems. However, this method severely limits the acquisition time of the imager and limits its use for many applications. Other methods, such as using antennas having beam directions that are a function of frequency, have also been implemented that require only a single row of RF LNAs. U.S. Pat. No. 5,365,237 to Johnson presents an example of this method of imaging.
The one-dimensional image acquisition methods provide savings in the cost of LNA units but increase the time needed for producing a full two-dimensional image.
Therefore, a need exists for millimeter wave imagers that include two-dimensional focal arrays at a reasonable cost, are capable of delivering useful performance at room temperature without requiring RF LNAs and RF Dicke switches, and have high responsivity with very low added noise. Embodiments of the present invention meet such a need.